The studies on thyroxine binding alpha-globulin have been extended and a report is in press describing the preparation and properties of the protein TBG. The thermal stability of this protein has been studied and its capacity to bind T4 has been shown to be inactivated by heating to 60 degrees C. Suggestive, but not conclusive, evidence of interaction between TBG and albumin has been obtained. Further studies have been undertaken regarding the metabolic significance of T3. Additional evidence of the extrathyroidal conversion of T4 to T3 has been obtained. This conversion has seemed to be appreciable in health and in most diseased conditions studied including thyroid diseases, but appears somewhat impaired in advanced Laennec's cirrhosis. In addition, preliminary studies with cell cultures in vitro have also signified conversion of T4 to T3. The factor of artefactual conversion of T4 to T3 in analytic systems has been under scrutiny as a limitation on the accuracy of the foregoing studies.